1. Field of the Invention
This invention relates to an ignition device for an internal combustion engine, and more particularly to a so-called "induction discharge type" ignition device which causes a spark at a spark plug induced by a high voltage in a secondary coil of an ignition coil when a current flowing through the primary coil of the ignition coil is cut off by a semiconductor power switching device.
2. Description of the Related Art
The invention in Japanese Patent Laid-Open No. 112630/1975 discloses that in induction discharge type ignition devices, the turns ratio of the primary winding to the secondary winding of the ignition coil must be made small and the inductance value on the side of the primary coil must be made sufficiently great in order to make the rise of the voltage occurring at the spark plug very steep and, moreover, to maintain the arc discharge over a long time period.
The secondary coil voltage V.sub.2 ' at the time of load is proportional to V.sub.z (breakdown voltage of the semiconductor power switching device) multiplied by the coil turns ratio a. Typically, V.sub.2 ' is 28 kV for a clean spark plug and the turns ratio, a is typically 85 to 100. However, there is a conflict since, because V.sub.2 ' is required to be high and the turns ratio, a, is required to be as low as possible, the semiconductor breakdown voltage V.sub.z is required to be increased but, as will be appreciated by those skilled in the art, there is a hardware limit as to how high the breakdown voltage can be made. Currently, the upper limit for the semiconductor power switching device, usually a Zener diode, is 400 V.
There is a further difficulty in that when the plug is in a smoldering condition, that is when the spark plug insulation is carbonised and wetted by gasoline, then there is breakdown between the outer, curved electrode and the insulation. The leakage path between the insulation and the curved outer electrode, for a clean spark plug, should, theoretically, be infinity, but is typically 10M.OMEGA.. However, when a spark plug is in the smoldering condition at low temperature of about -30.degree. C., the leakage resistance drops to about 100 k.OMEGA., which means that breakdown between the outer curved electrode and the insulation can occur at a voltage much lower than the normally operated 28 kV. It is believed to be a fundamental finding of the present applicants that the leakage resistance of the spark plug is in the range 100 k.OMEGA. to 10M.OMEGA. (effectively infinity).
There is a further problem that is encountered in the prior art that when an engine rotates at high speed, the spark generated between the outer curved electrode and the central electrode of the spark plug, the spark is blown out, that is briefly extinguished, by the stream of air-fuel mixture sucked into the cylinder. Thus, normal ignition is not effected during high speed revolutions of the engine. The present invention seeks to overcome the foregoing disadvantages associated with the prior art.